In a vented hydraulic fluid reservoir, air is allowed to pass over the surface of the hydraulic fluid. As the fluid level in the reservoir rises and falls due to factors such as system demands, fluid temperature, and evaporation, the reservoir breathes accordingly. To minimize the amount of abrasive contaminants from entering the reservoir all breathing is filtered. In such reservoir systems very small silting contaminants still get by the breather filter as do other contaminants in the form of a gas such as water vapor.
Water vapor that condenses in a reservoir froms water in the fluid. Water is one of the most dangerous contaminants to hydraulic fluids except water based hydraulic fluids. For example, its presence has a catalytic effect in the oxidation or decomposition of petroleum based hydraulic fluids. Oxidation of metal surfaces produces rust particles which when carried through the system are very detrimental.
When air or other gas is permitted to come in contact with a hydraulic fluid, it is very important that the gas does not aerate the fluid since aeration of the fluid is very detrimental to both the fluid and a pump in a high pressure system. This is due to the entrained air in the form of bubbles in the fluid being subjected to rapid changes in pressure from approximately atmospheric in the reservoir to maximum system pressure. A small air bubble in a hydraulic fluid which is rapidly compressed to 3,000 P.S.I., provides enough energy to raise the surface of the air bubble to 2,000.degree. F as it is compressed. This high local temperature results in cracking and nitration of the hydraulic fluid and is characteristic of the "burnt" odor present in cracked petroleum. A portion of the nitrogen from the air undoubtedly forms oxides which are highly reactive and attack both the hydrocarbon oil and any additive compounds. The many millions of air bubbles in the system will eventually breakdown the complete volume of hydraulic fluid. Another result of the compressing of many millions of air bubbles is the erosion of the pump components due to implosions of the voids combined with the high localized temperatures. This explains the outlet conditions of a pump when the hydraulic fluid is aerated. All material removed from the pump by this erosion process becomes an additional contaminant to the system which must be filtered out or it will generate more contaminants as it is forced through the system resulting in the system literally eating itself.
There have been attempts to eliminate the atmosphere from coming into direct contact with the hydraulic fluid by using a diaphragm or bladder type accumulator as the entire reservoir which results in an unsatisfactory reservoir since it prevents baffling the returned fluid, is limited to relatively small fluid connections, and makes for difficult filling of the system to the desired level.
This invention uses a contamination barrier which prevents atmospheric and gaseous contaminants from being admitted to the system via the systems' reservoir. It can be added to an existing reservoir and incorporated in new reservoir designs. It provides a means to fill the system and deaerate the fluid. It provides a means to inspect the fluid to determine if atmosphere is being pulled into the system from a defective joint or seal in a negative pressure leg in the system. It provides for a visual indication of the fluid level within the system, which permits monitoring controls to be easily adapted. It adds to the pressure head in the reservoir which improves the suction conditions and life of the pump. The improved suction conditions to the pump generally results in a quieter running pump. The invention permits operation with a very low differential pressure with large reservoir connection porting permitting high flow rates with minimal pressure drop which helps prevent turbulence.
By preventing foreign material from getting into the reservoir and by starting with a clean reservoir, it is possible in most systems to eliminate the pump's suction filter and use only pressure line and/or return line filtration in the system. Removing the suction filter further improves the pump's suction condition with its many benefits. In some systems the reduction in cost by eliminating the suction filter may pay for the contamination barrier with all its other benefits.